netdev - Re: [PATCH 2/2] tcp: fix undo after RTO for CUBIC

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Date:	Fri, 20 Jan 2012 14:18:35 -0500 (EST)
From:	David Miller <davem@...emloft.net>
To:	shemminger@...tta.com
Cc:	ncardwell@...gle.com, netdev@...r.kernel.org,
	ilpo.jarvinen@...sinki.fi, nanditad@...gle.com, ycheng@...gle.com,
	hkchu@...gle.com, therbert@...gle.com
Subject: Re: [PATCH 2/2] tcp: fix undo after RTO for CUBIC

From: Stephen Hemminger <shemminger@...tta.com>
Date: Thu, 19 Jan 2012 18:49:21 -0800

> On Wed, 18 Jan 2012 22:47:59 -0500
> Neal Cardwell <ncardwell@...gle.com> wrote:
> 
>> This patch fixes CUBIC so that cwnd reductions made during RTOs can be
>> undone (just as they already can be undone when using the default/Reno
>> behavior).
>> 
>> When undoing cwnd reductions, BIC-derived congestion control modules
>> were restoring the cwnd from last_max_cwnd. There were two problems
>> with using last_max_cwnd to restore a cwnd during undo:
>> 
>> (a) last_max_cwnd was set to 0 on state transitions into TCP_CA_Loss
>> (by calling the module's reset() functions), so cwnd reductions from
>> RTOs could not be undone.
>> 
>> (b) when fast_covergence is enabled (which it is by default)
>> last_max_cwnd does not actually hold the value of snd_cwnd before the
>> loss; instead, it holds a scaled-down version of snd_cwnd.
>> 
>> This patch makes the following changes:
>> 
>> (1) upon undo, revert snd_cwnd to ca->loss_cwnd, which is already, as
>> the existing comment notes, the "congestion window at last loss"
>> 
>> (2) stop forgetting ca->loss_cwnd on TCP_CA_Loss events
>> 
>> (3) use ca->last_max_cwnd to check if we're in slow start
>> 
>> Signed-off-by: Neal Cardwell <ncardwell@...gle.com>
> 
> I was unsure of this, so forwarded it to the author 
> and received his confirmation.
> 
> Acked-by: Stephen Hemminger <shemminger@...tta.com>
> Acked-by: Sangtae Ha <sangtae.ha@...il.com>

Applied.
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